Method and device for detecting physical bottom contact for objects on a sea bottom

ABSTRACT

A method and a device for detection of physical or approximately physical bottom contact for objects, for instance fishing tackles, which are dragged closely above or on a sea bottom, said method consisting of sensoring an alteration of a force acting on a tension sensor ( 4 ) as a consequence of a mass ( 7 ) coupled to the tension sensor ( 4 ) coming into physical contact with tha bottom, said alteration being used as indication of bottom contact, and that frictional forces (F 2 ) being due to the dragging of the mass ( 7 ) along the bottom mainly are taken up by means of a tension release means ( 8 ). The device comprise the mass ( 7 ) being suspended in a tension sensor ( 4 ) in such a manner that the sensor ( 4 ) is actuated by bottom contact, and at least one tension release means ( 8 ) connected to the mass ( 7 ) and adapted to take up forces caused by dragging of the mass above the bottom.

[0001] During fishing which takes place in close contact with or on a sea bottom it may be very important to know the distance between the tackle and the bottom.

[0002] For pelagic tackles this may be very important in order to avoid bottom contact, while for tackles constructed for fishing on the bottom, correct bottom contact will be crucial for the fishing efficiency.

[0003] A known technique is to use an echo sounder which is attached to the tackle and which measures the distance to the bottom. Very short distances which gradually lead to physical bottom contact may be difficult to detect with great reliability by use of an echo sounder, because it will be difficult to distinguish between echo from the tackle and the bottom in this phase. The use of an echo sounder for this purpose tends to be very complicated and expensive instrumentation.

[0004] It is previously known, from U.S. Pat. No. 4,873,523, to use a weight at the end of a rope which is kept taut when the weight is not in contact with the bottom, but when the weight hits the bottom, the rope will be slackened, and this is recorded by the surface vessel. This principle of detection is not reliable during dragging of a tackle above the bottom, because the frictional forces between the weight and the bottom may now and then cause tension in the rope.

[0005] The present invention eliminates this source of error.

[0006] The present invention relates to a method and a device for detecting physical or approximately physical bottom contact for objects, for instance fishing tackles, which are dragged closely above or on a sea bottom.

[0007] The method is characterized by sensoring an alteration of a force acting on a tension sensor as a consequence of a mass coupled to the tension sensor coming into physical contact with the bottom, said alteration being used as indication of bottom contact, and that frictional forces due to dragging of the mass along the bottom mainly are taken up by means of a tension release means.

[0008] The device is characterized in a mass being suspended in a tension sensor in such a manner that the sensor is actuated by bottom contact, and at least one tension release means connected to the mass and adapted to take up forces caused by dragging the mass above the bottom.

[0009] Thus, frictional forces which occur when the mass is dragged above the bottom are taken up by the tension release means and do not influence the tension sensor of the bottom detector.

[0010] Data from the tension sensor can be detected by a signalling device and transmitted to a vessel, for instance by means of hydroacoustical telemetry or through a cable connection.

[0011] The invention will be explained more detailed by means of the accompanying Figs., which illustrate examples of the invention.

[0012]FIG. 1 shows a sketch of a portion of a fishing tackle, shown in the form of a trawl that is towed slightly above the bottom. To a bottom rope 1 is attached a trawl net 5 and balls which constitute a part of the trawl. In this case it is desired to detect when the distance d between ball and bottom is less than a certain value or that the ball 9 is in physical contact with the bottom. A bottom detector 3 is attached to the trawl, preferably the bottom rope 1 and the trawl net 5, by means of a fastener device 2, whereby the detector 3 is kept i a stable horizontal position. To a tension sensor 4 is attached a measured length a-b of chain, wire, rope or similar, in the following called chain, 6. A measured length b-c of chain, wire, rope or similar, in the following called tension release means 8, is attached to for instance a bottom rope 1, preferably in the same point of attachment as the forward fastener device 2 of the bottom detector 3. Thereby, the chain 6 and the tension release means 8 have a total length a-b-c. A mass is attached to the chain 6 in the point b, in the form of a weight having a weight adapted to the measuring range of the tension sensor 4. The length a-b of the chain 6 determines the distance being desired to detect, and the length shall be such that the weight 7 is hanging freely and the force F1 is at a maximum when the weight is not in contact with the bottom. When contact with the bottom occurs, the force F1 is reduced, which is detected by the tension sensor 4.

[0013] Release of the weight of the weight 7 and a corresponding reduction of the force F1 when the weight touches the bottom determines the degree of bottom contact. The length b-c of the tension release means 8 is adapted in such a manner that the horizontal force component F2 caused by friction between the weight and the bottom is transferred to the bottom rope 1 through the tension release means 8 as soon as the weight 7 starts touching the bottom. FIG. 1 shows a situation where the weight is hanging straightly down, the triangle a-b-c is a right-angled triangle and F2 is at a maximum.

[0014] In order to safeguard the sensor against overloading, the chain 6 is attached to the tension sensor through a weak joint, for instance a rope lashing, adapted to burst before the tension sensor 4 is subjected to damage.

[0015]FIG. 2 shows a sketch of a portion of a fishing tackle being dragged on the bottom, i.e. having bottom contact. From the point of time when the weight 7 comes into physical contact with the bottom the force F1 will be reduced, and this is recorded with the bottom detector 3, which sends a signal up to the vessel. If the distance is further reduced, all the frictional forces between the bottom and the weight 7 will gradually be transferred to the bottom rope 1, F2 comes to a maximum and F1 comes correspondingly to a minimum. This is detected by the tension sensor 4, which sends a signal about maximum bottom contact up to the vessel. By transferring to the bottom rope 1 most of or all the frictional forces caused by dragging the weight 7 on the bottom, instead of to the tension sensor 4, measurement errors and erroneous interpretations will be avoided.

[0016]FIG. 3 shows a principal sketch of the tension sensor 4, having in mind sealing against water between the sensor element and the signalling device. The chain 6 is attached to a sensor strap 13 which influences the position of a permanent magnet 11. The magnet is biased by a mechanical spring 12, and the spring force can be adapted to the weight being used. The position of the magnet is read by a sensor, for instance a Reed relay 10, which in turn gives a signal to a signalling device 9 sending information about bottom contact or not to the vessel. The Reed relay may be replaced by a detector which detects the absolute position of the tension sensor, in order to indicate the degree of bottom contact.

[0017] The bottom detector 3, or a plurality thereof, may be attached anywhere on the fishing tackle or the object being dragged above or on the bottom.

[0018] The invention is not limited to the examples, and the use goes beyond the use on fishing tackles, for instance on towed or free floating underwater vehicles. 

1. A method for detection of physical or approximately physical bottom contact for objects, for instance fishing tackles, which are dragged closely above or on a sea bottom, characterized by sensoring an alteration of a force acting on a tension sensor (4) as a consequence of a mass (7) coupled to the tension sensor (4) coming into physical contact with the bottom, said alteration being used as indication of bottom contact, and that frictional forces (F2) being due to the mass (7) being dragged along the bottom mainly are taken up by means of a tension release means (8).
 2. A device for detection of physical or approximately physical bottom contact for objects, for instance fishing tackles, which are dragged closely above or on a sea bottom, characterized by a mass (7) being suspended in a tension sensor (4) in such a manner that the sensor (4) is actuated by bottom contact, and at least one tension release means (8) connected to the mass (7) and adapted to take up forces caused by dragging of the mass above the bottom.
 3. A device according to claim 2, in which the mass (7) is suspended in a chain (6) constituting both the tension release means (8) and suspension (6) at the sensor.
 4. A device according to claim 3, in which the suspension point of the mass is adapted to be shifted along the chain or similar (6, 8).
 5. A device according to one or more of the preceding claims, in which the suspension (6) of the mass is coupled to the tension sensor (4) through a weak coupling element.
 6. A device according to one or more of the preceding claims, in which signals from the tension sensor (4) can be acoustically transferred to a device on the tackle and/or directly up to the vessel. 